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Torrents and droughts and twisters - oh my!

How much is extreme weather affected by greenhouse gases?

Not all kinds of extreme weather have the same relationship with our atmosphere's increasing burden of greenhouse gas. Below is a summary of what scientists already know and what they're working to nail down, including some conclusions from recent reports by the Intergovernmental Panel on Climate Change (see What is the IPCC?).

Click on each label or image to learn more about that phenomenon's links to climate change. For help with the terminology used in IPCC statements, see the quick guide at bottom.

Observations collected atop Hawaii's Mauna Loa since the late 1950s, and at many other locations around the world, confirm that the amount of carbon dioxide in Earth's atmosphere is increasing every year. Now about 390 parts per million (ppm), the concentration has risen by more than 30% since preindustrial times and continues to grow by around 1 to 3 ppm per year. Each year's increase is influenced by economic activity (less CO2 is added when a recession is underway) as well as by natural and human-induced factors, including processes such as El Niño and La Niña, that affect the total amount of vegetation consuming CO2 through growth or releasing it through fire in a given year. Other greenhouse gases, such as methane (CH4) and nitrous oxide (N2O) are also increasing.

The atmospheric concentrations of CO2 and CH4 in 2005 exceed by far the natural range over the last 650,000 years. Global increases in CO2 concentrations are due primarily to fossil fuel use, with land-use change providing another significant but smaller contribution.

It is very likely that the observed increase in CH4 concentration is predominantly due to agriculture and fossil fuel use. The increase in N2O concentration is primarily due to agriculture.

The carbon dioxide emitted by fossil fuels mixes throughout Earth's atmosphere, and its effect is most noticeable on temperatures averaged worldwide. A wide range of studies since the 1990s have confirmed that the bulk of global warming over the last few decades, as well as its regional characteristics, or "fingerprint"—such as greater warming observed at higher latitudes—can be attributed to human-produced greenhouse gases.

Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic GHG concentrations. It is likely that there has been significant anthropogenic warming over the past 50 years averaged over each continent (except Antarctica).

Even a small rise in average temperature can substantially boost the odds of extreme heat—and reduce the odds of extreme cold—by pushing the ends of the temperature spectrum beyond certain thresholds, as shown in this graphic.

When scientists attempt to link a particular heat wave to human factors, the results have been mixed, partly due to the regions examined and how the question is posed. One major study found it is very likely that human influence at least doubled the odds of an event in Europe like the catastrophic 2003 heat wave. Another group found that recent trends did not explain the 2010 heat wave in western Russia, though they added that such events should become more likely later this century as greenhouse gases accumulate.

It is very likely that there has been an overall decrease in the number of cold days and nights, and an overall increase in the number of warm days and nights, on the global scale, i.e., for most land areas with sufficient data.

It is likely that these changes have also occurred at the continental scale in North America, Europe, and Australia.

There is medium confidence of a warming trend in daily temperature extremes in much of Asia. Confidence in observed trends in daily temperature extremes in Africa and South America generally varies from low to medium depending on the region.

In many (but not all) regions over the globe with sufficient data there is medium confidence that the length or number of warm spells, or heat waves, has increased.

It is likely that anthropogenic influences have led to warming of extreme daily minimum and maximum temperatures on the global scale.

Warmer temperatures allow for more evaporation from oceans and lakes. The added moisture in the air could help intensify rain and snow where it's falling. Since the 1970s, many regions—including the United States—have seen more precipitation clustered into the heaviest rain and snow events.

Analysts have noted that this global picture is consistent with what one would expect from climate change. However, not all regions follow these trends, and regional-scale attribution is challenging because precipitation is difficult to measure (for reasons given in the IPCC excerpt below) and it can vary greatly over small distances. Moreover, the impact of heavier rain and snow on flooding depends in part on how cities and waterways are structured. A recent study found that England's flooding of 2000 was made at least 20% more likely by human influence.

There have been statistically significant trends in the number of heavy precipitation events in some regions. It is likely that more of these regions have experienced increases than decreases, although there are strong regional and subregional variations in these trends.

There is limited to medium evidence available to assess climate-driven observed changes in the magnitude and frequency of floods at regional scales because the available instrumental records of floods at gauge stations are limited in space and time, and because of confounding effects of changes in land use and engineering. Furthermore, there is low agreement in this evidence, and thus overall low confidence at the global scale regarding even the sign of these changes.

There is medium confidence that anthropogenic influences have contributed to intensification of extreme precipitation on the global scale.

Warmer temperatures not only allow more evaporation from bodies of water (see "Intense rain or snow"), they also help draw moisture from already-dry soils. All else being equal, this would help strengthen the effects of drought on areas that are parched. In line with this concept, the global spread of drought has increased with global temperatures in the second half of the 20thcentury.

As with precipitation, there are large regional variations in this picture. It can be difficult to untangle the combined effects of heat and drought and determine how much climate change might have played a role in each one. Large-scale ocean patterns, such as El Niño/La Niña and the Pacific Decadal Oscillation, play a major role in shaping drought, and it's not yet clear how climate change will affect these.

There is medium confidence that some regions of the world have experienced more intense and longer droughts, in particular in southern Europe and West Africa, but in some regions droughts have become less frequent, less intense, or shorter, e.g., in central North America and northwestern Australia.

Over the last decade, researchers have vigorously debated the effect of increased greenhouse gases on recent and future trends in tropical cyclones, including North Atlantic hurricanes. Hurricanes draw energy from warm water, and sea surface temperatures (SSTs) have risen across many hurricane-prone areas in recent decades, with greenhouse gases likely a factor. Also, the number and power of the strongest Atlantic hurricanes has grown since the 1970s in line with SSTs. But hurricane records are limited for the period before satellites and hurricane-hunter flights, complicating the research. Moreover, future changes in wind shear may partially counteract the nourishing effect of warming oceans on hurricanes in some areas, including the North Atlantic.

According to a 2010 paper by an all-star group of hurricane researchers, scientific knowledge of the physics of hurricanes and computer modeling studies agree that Earth as a whole will shift during this century toward fewer but stronger tropical cyclones. Their impact on society will depend largely on which hurricanes strike land (a function of short-term weather) and how much population and construction occurs in coastal areas.

There is low confidence in any observed long-term (i.e., 40 years or more) increases in tropical cyclone activity (i.e., intensity, frequency, duration), after accounting for past changes in observing capabilities.

The uncertainties in the historical tropical cyclone records, the incomplete understanding of the physical mechanisms linking tropical cyclone metrics to climate change, and the degree of tropical cyclone variability provide only low confidence for the attribution of any detectable changes in tropical cyclone activity to anthropogenic influences.

A unique blend of large-scale weather conditions is required to support the long-lived supercell thunderstorms that produce the most dangerous tornadoes. Although the number of observed U.S. tornadoes has more than doubled since the 1950s, as more spotters and chasers watch the skies, there has been no significant trend in the strongest twisters (EF3 or greater).

Recent work suggests that the instability fueling supercells may increase across much of the eastern U.S. this century, but the wind shear that supports tornadic storms may not. The upshot could be more severe weather (high wind gusts, heavy rain) without any increase in tornadoes. It is not yet clear whether any increase in nontornadic high winds would take the form of derechoes, such as the one that struck Washington, D.C., in June 2012. To get a better handle on how severe weather will evolve in our future climate, scientists are now embedding small-scale weather models that simulate thunderstorms in large-scale climate models that depict global warming.

There is low confidence in observed trends in small spatial-scale phenomena such as tornadoes and hail because of data inhomogeneities and inadequacies in monitoring systems.

Quick guide to terms used by the IPCC

Anthropogenic: caused by human activity

Data inhomogeneities: inconsistencies in the way the data was gathered or archived

GHG: greenhouse gas

Sign: When referring to change, a positive sign means a trend is increasing; a negative sign means the trend is decreasing

Likelihood ("likely," "very likely," etc.): see the guide to definitions used in the IPCC 2007 Fourth Assessment Report

Evidence, agreement, and confidence ("limited agreement," "medium evidence," etc.): see the table of definitions used in the IPCC 2011 special report on extremes (Box SPM.2, on the final page of this PDF).